navCtrl.push 在 Ionic3 中不工作

Question

我正在使用 Ionic 制作一个应用程序，将图像上传到我的 Firebase 存储。上传完成后，应用程序应将用户重定向到 WelcomePage。上传工作正常，但由于某种原因，用户没有被重定向。看一下代码的结尾：控制台确实显示 Uploaded，但是 this.navCtrl.push(WelcomePage) 不工作。

upload() {
  let storageRef = firebase.storage().ref();

  const filename = "profilePic";

  const imageRef = storageRef.child(`images/${filename}.jpg`).putString(this.profilepicture, firebase.storage.StringFormat.DATA_URL);

  imageRef.on('state_changed', function(snapshot){

    var progress = (snapshot.bytesTransferred / snapshot.totalBytes) * 100;
    document.getElementById("percent").innerHTML = "Progresso:" + Math.floor(progress) + "%";   
    console.log('Upload is ' + progress + '% done');
    switch (snapshot.state) {
      case firebase.storage.TaskState.PAUSED:
        console.log('Upload is paused');
        break;
      case firebase.storage.TaskState.RUNNING:
        console.log('Upload is running');
        break;
    }
  }, function(error) {
    // Handle unsuccessful uploads
  }, function() {
    // Handle successful uploads on complete
    console.log('Uploaded');
    this.navCtrl.push(WelcomePage);
  });

 }

Answer 1

很可能 this 的含义在您的回调中有所不同。如果是这种情况，您可以将 navCtrl 放在一个单独的变量中，或者您可以使用粗箭头函数。

将navCtrl放在一个单独的变量中

this 的含义取决于您在其中使用它的函数。由于回调是独立的函数，因此它们对 this 也有自己的值。所以在回调中 this.navCtrl 不存在。

解决方法是在回调之前将你需要的值捕获到一个唯一的变量名中：

var navCtlr = this.navCtrl;
imageRef.on('state_changed', function(snapshot){
  var progress = (snapshot.bytesTransferred / snapshot.totalBytes) * 100;
  document.getElementById("percent").innerHTML = "Progresso:" + Math.floor(progress) + "%";   
  console.log('Upload is ' + progress + '% done');
  switch (snapshot.state) {
    case firebase.storage.TaskState.PAUSED:
      console.log('Upload is paused');
      break;
    case firebase.storage.TaskState.RUNNING:
      console.log('Upload is running');
      break;
  }
}, function(error) {
  // Handle unsuccessful uploads
}, function() {
  // Handle successful uploads on complete
  console.log('Uploaded');
  navCtrl.push(WelcomePage);
});

使用粗箭头函数

ES6 添加了一种使用粗箭头表示法定义回调函数的新方法。在您的代码中 function(error) { 可以替换为 (error) => {。除了稍微更短之外，新符号实际上在回调中保留了 this 的含义。所以：

imageRef.on('state_changed', (snapshot) => {
  var progress = (snapshot.bytesTransferred / snapshot.totalBytes) * 100;
  document.getElementById("percent").innerHTML = "Progresso:" + Math.floor(progress) + "%";   
  console.log('Upload is ' + progress + '% done');
  switch (snapshot.state) {
    case firebase.storage.TaskState.PAUSED:
      console.log('Upload is paused');
      break;
    case firebase.storage.TaskState.RUNNING:
      console.log('Upload is running');
      break;
  }
},(error) => {
  // Handle unsuccessful uploads
}, () => {
  // Handle successful uploads on complete
  console.log('Uploaded');
  this.navCtrl.push(WelcomePage);
});