QGis（三）查询矢量图层的要素属性字段值

当加载一个矢量图层后，如果要查看要素的属性字段值，则需要实现identity的功能。可以和前面的缩放一样，添加一个工具栏按钮：

（1）在MainWindow添加一个变量，并在Qt设计师里添加Action：

QgsMapToolSelect *mpIdentifyTool;		///<查询要素

这里QgsMapToolSelect类继承自QgsMapTool，后面再列出详细内容。

（2）然后在初始化函数里添加：

mpMapToolBar->addAction(ui.mpActionIdentify);
mpIdentifyTool = new QgsMapToolSelect(mainMapCanvas);
mpIdentifyTool->setAction(ui.mpActionIdentify);

（3）添加信号槽函数连接：

connect(ui.mpActionIdentify, SIGNAL(triggered()), this, SLOT(identifyFeature()));

（4）槽函数实现：

void MainWindow::identifyFeature()
{
mpIdentifyTool->SetEnable(true);
mainMapCanvas->setMapTool(mpIdentifyTool);
ui.mpActionIdentify->setCheckable(true);
ui.mpActionIdentify->setChecked(true);
if ( mainMapCanvas->layer(0)->type() == QgsMapLayer::RasterLayer)
{
return;
}
QgsVectorLayer *pLayer=(QgsVectorLayer *)mainMapCanvas->layer(0);
mpIdentifyTool->SetSelectLayer(pLayer);
if ( ui.mpActionIdentify->isChecked())
{
pLayer->removeSelection(true);
}
}

可以看到，基本步骤就跟前面的缩放工具栏一模一样。但是QgsMapToolSelect不是QgsMapTool自带的类，需要自定义，QgsMapToolSelect类的定义如下：

class QgsMapToolSelect :
public QgsMapTool
{
Q_OBJECT;
public:
QgsMapToolSelect(QgsMapCanvas *);
~QgsMapToolSelect(void);
public:
//设置当前被选择(活动)的图层
void SetSelectLayer(QgsVectorLayer *);
<span style="white-space:pre">	</span>//重载鼠标释放事件函数
virtual void canvasReleaseEvent(QMouseEvent * e);
//设定工具状态
void SetEnable(bool);
//得到所有的属性和属性值，<属性1，属性值1>是QMap的一对值
QList<QMap<QString, QString>> GetAttributeValue(QgsVectorLayer *layer, QgsFeatureIds selectedFIds);
void setShowFlag(bool flag);
IdentifyResultDlg *ResultDlg();
private:
QgsVectorLayer* pLayer;
QgsFeatureIds layerSelectedFeatures;
bool StatusFlag;
QList<QMap<QString, QString>> list; //存储选择后的要素
bool isShowFlag; ///< 是否在主界面上勾选了shp图层
IdentifyResultDlg *mIdentifyResultDlg;

private:
<span style="white-space:pre">	</span>//提取鼠标位置一定范围作为选择区域
void ExpandSelectRangle(QRect &Rect,QPoint Point);
//将指定的设备坐标区域转换成地图坐标区域
void SetRubberBand(QRect &selectRect,QgsRubberBand *);
//选择图层特征
void SetSelectFeatures(QgsGeometry *,bool,bool,bool);
void SetSelectFeatures(QgsGeometry *,bool);
};

具体函数实现：

#include "qgsmaptoolselect.h"

//构造和析构函数
QgsMapToolSelect::QgsMapToolSelect(QgsMapCanvas *Mapcanvas):QgsMapTool(Mapcanvas)
{
pLayer=NULL;
mCursor=Qt::ArrowCursor;
mCanvas=Mapcanvas;
StatusFlag=true;
isShowFlag = true;
mIdentifyResultDlg = NULL;
}

QgsMapToolSelect::~QgsMapToolSelect(void)
{
}
//设置当前被选择(活动)的图层
void QgsMapToolSelect::SetSelectLayer(QgsVectorLayer *Layer)
{
pLayer=Layer;
}

//鼠标按钮释放时,选择包含鼠标位置的图元
void QgsMapToolSelect::canvasReleaseEvent(QMouseEvent * e )
{
if(mCanvas==NULL){
return;
}
if(pLayer==NULL){
QMessageBox::about(mCanvas,QString::fromLocal8Bit("警告"),QString::fromLocal8Bit("请选择图层"));
return;
}
if(StatusFlag==false){
return;
}
//得到产生事件的按钮信息
Qt::MouseButton mButton=e->button();
//如果不是左按钮返回
if(mButton!=Qt::MouseButton::LeftButton){
return;
}
//得到鼠标指针的位置
QPoint pos=e->pos();
//定义QgsRubberBand对象
QgsRubberBand rubberBand(mCanvas,true);
QRect selectRect(0,0,0,0);
//设置鼠标位置区域
ExpandSelectRangle(selectRect,pos);
//将鼠标位置区域转换成地图坐标
SetRubberBand(selectRect,&rubberBand );
//将QgsRubberBand对象转换为几何对象，根据该几何对象在图层中选择特征
QgsGeometry* selectGeom=rubberBand.asGeometry();
if(!selectGeom){
return;
}
//确定是否按下ctrl键
bool doDifference=e->modifiers()&Qt::ControlModifier ? true:false;
//在图层中选择最靠近几何对象的特征
SetSelectFeatures(selectGeom,false,doDifference,true);
//SetSelectFeatures(selectGeom,doDifference);

//设定选择的特征
pLayer->setSelectedFeatures(layerSelectedFeatures);
list = GetAttributeValue(pLayer, layerSelectedFeatures);

if (isShowFlag)
{
ResultDlg()->InitialData(list);
ResultDlg()->show();
}
else
{
QMessageBox::information(NULL, tr("警告"), tr("请选择矢量图层"), QMessageBox::Ok);
}
delete selectGeom;
rubberBand.reset(true);

}
//提取鼠标位置一定范围作为选择区域
void QgsMapToolSelect::ExpandSelectRangle(QRect &Rect,QPoint Point)
{
int boxSize=0;
//如果图层不是面图元类型
if(pLayer->geometryType()!=QGis::Polygon){
boxSize=5;
}
else{
boxSize=1;
}
//设置选择区域
Rect.setLeft(Point.x()-boxSize);
Rect.setRight(Point.x()+boxSize);
Rect.setTop(Point.y()-boxSize);
Rect.setBottom(Point.y()+boxSize);
}
//将指定的设备坐标区域转换成地图坐标区域
void QgsMapToolSelect::SetRubberBand(QRect &selectRect,QgsRubberBand *pRubber)
{
//得到当前坐标变换对象
const QgsMapToPixel* transform=mCanvas->getCoordinateTransform();
//将区域设备坐标转换成地图坐标
QgsPoint ll=transform->toMapCoordinates(selectRect.left(),selectRect.bottom());
QgsPoint ur = transform->toMapCoordinates(selectRect.right(),selectRect.top());
pRubber->reset(true );
//将区域的4个角点添加到QgsRubberBand对象中
pRubber->addPoint(ll,false );
pRubber->addPoint(QgsPoint(ur.x(), ll.y()),false );
pRubber->addPoint(ur,false );
pRubber->addPoint(QgsPoint( ll.x(), ur.y() ),true );
}
//选择几何特征
//selectGeometry:选择特征的选择几何体
//doContains：选择的特征是否包含在选择几何体内部
//singleSelect：仅仅选择和选择几何体最靠近的特征
void QgsMapToolSelect::SetSelectFeatures(QgsGeometry *selectGeometry,bool doContains,
bool doDifference,bool singleSelect)
{
//如果选择几何体不是多边形
if(selectGeometry->type()!=QGis::Polygon){
return;
}
QgsGeometry selectGeomTrans(*selectGeometry);
//设定选择几何体的坐标系和图层的坐标系一致
if(mCanvas->mapRenderer()->hasCrsTransformEnabled()){
try{
//将地图绘板坐标系变换到图层坐标系
QgsCoordinateTransform ct(mCanvas->mapRenderer()->destinationSrs(),pLayer->crs());
//设定几何体的坐标系和图层坐标系一致
selectGeomTrans.transform(ct);
}
//对于异常点抛出异常
catch(QgsCsException &cse){
Q_UNUSED(cse);
//catch exception for 'invalid' point and leave existing selection unchanged
QMessageBox::warning(mCanvas, QObject::tr("CRS Exception"),
QObject::tr( "Selection extends beyond layer's coordinate system." ) );
return;
}
}
//设置光标
//QApplication::setOverrideCursor(Qt::WaitCursor);
//选择和选择几何体相交或在几何体内部的特征
pLayer->select(QgsAttributeList(),selectGeomTrans.boundingBox(),true,true);
int nh=pLayer->selectedFeatureCount();
QgsFeatureIds newSelectedFeatures;
QgsFeature f;
int closestFeatureId=0;
bool foundSingleFeature=false;
double closestFeatureDist=std::numeric_limits<double>::max();
//得到当前选择的特征
while(pLayer->nextFeature(f)){
QgsGeometry* g=f.geometry();
//g是否包含在selectGeomTrans几何体内部
if(doContains && !selectGeomTrans.contains(g)){
continue;
}
if(singleSelect){ //选择和几何体最靠近的特征
foundSingleFeature=true;
//计算两个几何体之间的距离
double distance=g->distance(selectGeomTrans);
if(distance<=closestFeatureDist){
closestFeatureDist=distance;
//计算出最靠近选择几何体特征的id
closestFeatureId=f.id();
}
}
else{ //存储符合要求的特征id
newSelectedFeatures.insert(f.id());
}
}
//确定和选择几何体最靠近特征的id
if(singleSelect && foundSingleFeature){
newSelectedFeatures.insert(closestFeatureId);
}
//如果按下ctrl键,选择多个特征
if(doDifference){
//得到所有选择特征的id
layerSelectedFeatures=pLayer->selectedFeaturesIds();
QgsFeatureIds::const_iterator i=newSelectedFeatures.constEnd();
while(i!=newSelectedFeatures.constBegin()){
--i;
if(layerSelectedFeatures.contains(*i)){
layerSelectedFeatures.remove( *i );
}
else{
layerSelectedFeatures.insert( *i );
}
}
}
else{
layerSelectedFeatures=newSelectedFeatures;
}

//QApplication::restoreOverrideCursor();
}
//选择几何特征,用于选择面状几何特征
//selectGeometry:选择几何体
void QgsMapToolSelect::SetSelectFeatures(QgsGeometry *selectGeometry,bool doDifference)
{
//如果选择几何体不是多边形
if(selectGeometry->type()!=QGis::Polygon){
return;
}
QgsGeometry selectGeomTrans(*selectGeometry);
//设定选择几何体的坐标系和图层的坐标系一致
if(mCanvas->mapRenderer()->hasCrsTransformEnabled()){
try{
//将地图绘板坐标系变换到图层坐标系
QgsCoordinateTransform ct(mCanvas->mapRenderer()->destinationSrs(),pLayer->crs());
//设定几何体的坐标系和图层坐标系一致
selectGeomTrans.transform(ct);
}
//对于异常点抛出异常
catch(QgsCsException &cse){
Q_UNUSED(cse);
//catch exception for 'invalid' point and leave existing selection unchanged
QMessageBox::warning(mCanvas, QObject::tr("CRS Exception"),
QObject::tr( "Selection extends beyond layer's coordinate system." ) );
return;
}
}
//设置光标
//QApplication::setOverrideCursor(Qt::WaitCursor);
//选择和选择几何体相交或在几何体内部的特征
pLayer->select(QgsAttributeList(),selectGeomTrans.boundingBox(),true,true);
int nh=pLayer->selectedFeatureCount();
QgsFeatureIds newSelectedFeatures;
QgsFeature f;

int  p=0;
//得到当前选择的特征
while(pLayer->nextFeature(f)){
p++;
QgsGeometry* g=f.geometry();
//选择的特征是否包含在选择几何体的内部
//如果g包含selectGeomTrans，返回true
if(!g->contains(&selectGeomTrans)){
continue;
}
//存储符合条件图层特征id
newSelectedFeatures.insert(f.id());
}
QgsFeatureIds layerSelectedFeatures;
//如果按下ctrl键，可以选择多个特征
if(doDifference){
layerSelectedFeatures=pLayer->selectedFeaturesIds();
QgsFeatureIds::const_iterator i = newSelectedFeatures.constEnd();
while( i != newSelectedFeatures.constBegin()){
--i;
if( layerSelectedFeatures.contains( *i ) )
{
layerSelectedFeatures.remove( *i );
}
else
{
layerSelectedFeatures.insert( *i );
}
}
}
else{
layerSelectedFeatures=newSelectedFeatures;
}
//设定选择的特征
pLayer->setSelectedFeatures(layerSelectedFeatures);
//QApplication::restoreOverrideCursor();
}
//设定工具状态
void QgsMapToolSelect::SetEnable(bool flag)
{
StatusFlag=flag;
if(StatusFlag){
mCursor=Qt::CrossCursor;
}
else{
mCursor=Qt::ArrowCursor;
}
}

IdentifyResultDlg * QgsMapToolSelect::ResultDlg()
{
if (mIdentifyResultDlg == NULL)
{
mIdentifyResultDlg = new IdentifyResultDlg;
}
return mIdentifyResultDlg;
}

//得到选择的要素的属性信息
QList<QMap<QString, QString>> QgsMapToolSelect::GetAttributeValue( QgsVectorLayer *layer, QgsFeatureIds selectedFIds )
{
//	QStringList strAttribute;
QMap<QString, QString> featureValue;
QList<QMap<QString, QString>> featureValues;
int i = 0, nFeatureCount = 0, nFieldsCount = 0;
if (layer == NULL)
{
return featureValues;
}
QgsFeatureList featurelist = layer->selectedFeatures();
nFeatureCount = featurelist.size();
QgsFeature feature;
QString fieldName, fieldValue;
//	vector<QgsField> myFields = layer->fields();
for (i = 0; i < nFeatureCount; i++)
{
feature = featurelist.at(i);
const QgsAttributeMap &attributes = feature.attributeMap();
nFieldsCount = attributes.count();
const QgsFieldMap &fields = layer->pendingFields();//从图层得到字段信息
QgsField field;
for(int j = 0; j < nFieldsCount; j++)
{
field = fields[j];
fieldName = field.name();

fieldValue = attributes[j].toString();
featureValue.insert(fieldName, fieldValue);
}
featureValues.push_back(featureValue);
}
return featureValues;
}

void QgsMapToolSelect::setShowFlag( bool flag )
{
isShowFlag = flag;
}

查询得到的数据显示在对话框：

IdentifyResultDlg对话框是为了显示查询得到的结果。它的定义：

class IdentifyResultDlg : public QDialog
{
Q_OBJECT

public:
IdentifyResultDlg(QWidget *parent = 0);
~IdentifyResultDlg();
void InitialDlg();
void InitialData(QList<QMap<QString, QString>>);
private:
Ui::IdentifyResultDlg ui;
QStandardItemModel *fieldmodel;

};

IdentifyResultDlg具体实现：

#include "identifyresultdlg.h"
IdentifyResultDlg::IdentifyResultDlg(QWidget *parent)
: QDialog(parent)
{
ui.setupUi(this);
QStringList lists;
InitialDlg();
}
IdentifyResultDlg::~IdentifyResultDlg()
{
}
void IdentifyResultDlg::InitialDlg()
{
this->setWindowTitle(tr("查询要素属性信息"));
fieldmodel = new QStandardItemModel();
fieldmodel->setColumnCount(2);

ui.tableView->setModel(fieldmodel);
ui.tableView->setEditTriggers(QAbstractItemView::NoEditTriggers);//设置表格不可编辑
ui.tableView->verticalHeader()->hide(); //列头不显示
ui.tableView->setSelectionBehavior(QAbstractItemView::SelectRows);//选择整行高亮
setWindowFlags(Qt::WindowStaysOnTopHint);//使窗口置顶
this->resize( 240, 300);
}
//将数据显示在表格中
void IdentifyResultDlg::InitialData( QList<QMap<QString, QString>> datas)
{
fieldmodel->clear();//注意：在clear之后，表头名称也被清除了，需再设置表头
fieldmodel->setHorizontalHeaderItem(0, new QStandardItem(QObject::tr("属性名称")));
fieldmodel->setHorizontalHeaderItem(1, new QStandardItem(QObject::tr("属性值")));

QString name, value;
QMap<QString,QString>::iterator it;
int i = 0;
for (i = 0; i < datas.count(); i ++)
{
QMap<QString,QString> data = datas.at(i);
int j = 0;
for (it = data.begin(),  j = 0; it != data.end(), j < data.count(); ++it, ++j)
{
int index = i * data.count() +j;
fieldmodel->setItem(index, 0, new QStandardItem(it.key()));
if (it.value() == NULL)
{
it.value() = "0";
}
fieldmodel->setItem(index, 1, new QStandardItem(it.value()));
fieldmodel->item(index, 0)->setTextAlignment(Qt::AlignCenter);
fieldmodel->item(index, 1)->setTextAlignment(Qt::AlignCenter);
if ( j%(data.count()) == 0)
{
fieldmodel->item(index, 0)->setBackground(QBrush(QColor(188, 188, 188)));
fieldmodel->item(index, 1)->setBackground(QBrush(QColor(188, 188, 188)));
}
}
}
}

查询一个要素和多个要素的结果如下图所示：





注意：可能中文字段显示乱码，解决方法：

在操作shp之前，添加这句就可以了：  

CPLSetConfigOption("SHAPE_ENCODING","");